A typical CT scanner includes an x-ray tube that emits radiation from a focal spot in a direction of an examination region. A source collimator is disposed between the focal spot and the examination region and collimates the emitted radiation to produce a beam having a pre-determined geometrical shape (e.g., fan, cone, wedge, etc.). The collimated beam traverses the examination region and a portion of an object or subject therein (which attenuates the beam as a function of the radiodensity of the object or subject) and illuminates a detector array disposed across the examination region, opposite the x-ray tube. The detector produces projection data indicative of the detected radiation. The projection data can be reconstructed to generate volumetric image data indicative of the portion of the object or subject.
A pre-patient radiation filter (often referred to as a bow-tie filter due to its shape) has been positioned between the focal spot and the collimator to spatially attenuate the emitted radiation to shape the transmission profile. FIG. 1 schematically illustrates an example of a bow-tie filter 102 in connection with a focal spot 104, a source collimator 106, an x-ray beam 108, a detector array 110, an examination region 112, and a portion of a subject or object 114 therein. Due to its shape, the bow-tie filter 102 heavily filters regions of the beam 108 that traverse only air, lightly filters the region of the beam 108 that traverses the subject 114, and varies the degree of filtering for the transitions there between. FIG. 2 illustrates an example resulting transmission profile 200 as a function of beam angle, where a y-axis 202 represents transmission and an x-axis 204 represents beam angle. Note that the profile 200 varies as a function of the thickness of filter 102.
Unfortunately, the bow-tie filter 102 also preferentially filters lower energy rays relative to higher energy rays, thereby changing the x-ray spectrum of the beam exiting the filter 102, relative to the beam entering the filter 102. As such, the spectrum of the beam used to scan a subject or object may not be the optimal or desired spectrum. Furthermore, x-rays that are scattered by the bow-tie filter 102 degrade image quality and may require image processing scatter corrections to mitigate. Scatter from the bow-tie filter 102 may also contribute to patient dose while not contributing to diagnostic information in the reconstructed images. Furthermore, a geometry of the bow-tie filter 102 defines a minimum spacing between the focal spot 104 and the collimator 106, which may limit the size of the examination region 112 for a given focal spot location and/or require moving the x-ray tube for a given examination region size.
In view of at least the above, there is an unresolved need for other approaches for shaping the transmission profile of the radiation beam of a CT scanner.